DC current sensor

Abstract: It is an object of the present invention to provide a DC current sensor having, a relatively simple construction and a good detecting capability even for microscopic current variation such as DC leakage breaker and the like, particularly, it is an object to provide a DC current sensor which is versatile and easy to mount to a wired lead. A lead wire being detected 1 is disposed through a strip type detecting core 2 constituted by, an integrated U-shaped detecting core member 2b, I-shaped detecting core members 21a,21b and opposite portions 52a,52b,52c,52d of an exciting core 4 consisting of four pieces of substantially rectangular cylindrical core members 4a,4b,4c,4d disposed oppositely via the detecting core members 21a,21b, whereby, against a magnetic flux Φ produced in the detecting core 2 by a DC current flowing through the lead wire 1, by using a magnetic flux, which is produced by an AC current of frequency f₀ in existing coils 5a,5b wound around adjoining portions 51a,51b and 51c,51d of the exciting core members 4a,4b,4c,4d for negative-phase excitation, a magnetic gap is formed periodically by the opposite portions 52a,52b,52c,52d forming a core intersection of the exciting core, which is connected substantially perpendicularly to a circumferential direction of the detecting core members, to modulate the magnetic flux Φ₀ by 2f₀ for detecting an electromotive force proportional to the DC current I flowing through the lead wire being detected 1 from the detecting coil 3 wound around the exciting core 4.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a DC current sensor used in a DC leakage breaker and the like, and, particularly, it relates to a highly versatile DC current sensor, having a relatively simple construction and a good detecting capability against microscopic current variation, which can be mounted very easily to a lead wire for realizing a stable detection. Description of the Prior Art In recent years, many types of equipment have been developed which use a DC current such as electric appliances incorporating an inverter, electromobiles and the like. Such equipment requires for control a sensor which detects the load of DC motors built in the various equipments. Current sensors used in AC leakage breakers and the like, which include a current transformer are widely known. However, this configuration can not be adopted for use in a leakage breaker used in the aforementioned DC current equipment, and hence, a conventionally known shunt resistance method, magnetic amplifier method, magnetic multi-vibrator method (Japanese Patent Application Laid Open Nos. Sho 47-1644, Sho 53-31176, Sho 59-46859), hall device and the like have been studied for these applications. The shunt resistance method is a method for detecting a potential difference produced at opposite end portions of the shunt resistance which is disposed in series with a lead wire. In the magnetic amplifier method and the magnetic multi-vibrator method, a core made of a soft magnetic material constituted by winding a detecting coil in a toroidal shape is used, wherein the lead wire is extended through the core. A DC magnetic deflection of the core, made of a soft magnetic material, within a saturated magnetic flux density (Bs), is caused by the DC current flowing through the lead wire. In the magnetic amplifier method, in order to give the magnetic flux variation in the core in advance, an exciting coil is wound on the core to apply the AC current having a predetermined value. In the magnetic multi-vibrator method, the self-oscillation takes place by the operation of a semiconductor and the like in a circuit connected to the detecting coil, and a duty ratio of the oscillation waveform is changed responsive to the electric current being detected for oscillation. Furthermore, in the hall device method, the lead wire is wound in a toroidal shape directly around the core made of soft magnetic material, whereon a gap portion for disposing the hall device is formed partly. The magnetic flux variation in the core responsive to the variation in the DC current flowing through the lead wire is detected directly by the hall device. However, DC current sensors of these types do not respond well to microscopic current variations of the DC leakage breaker and the like for the following reasons, and at present, they are not practical to use as a sensitive DC current sensor. That is, in the shunt resistance method, since the shunt resistance itself is disposed, as an electrical resistances, in a circuit including the lead wire, an electrical loss in the circuit increases and electrical efficiency is deteriorated. Likewise, since a detecting circuit for detecting the potential difference produced at opposite ends of the electrical resistance is directly connected to the lead wire it is difficult to insulate the detecting circuit from the lead wire. For example, the detecting circuit and an applied circuit such as a microcomputer control circuit, and the like can not be connected directly, which results in a poor versatility. As well as these disadvantages, since two shunt resistances must be disposed in the circuit of the lead wire in order to adopt the shunt resistance method in the leakage breaker, it is, in fact, difficult to make the characteristics of the respective shunt resistances uniform. Thus a precise measurement of the potential difference can not be realised. Also, in order to detect a small leakage by comparing the potential difference measured by the detecting circuit connected to the respective shunt resistances, it is necessary to connect the respective detecting circuits through a very complicated electric circuit. Thus it is difficult to provide a highly practical DC current sensor of this type. In the magnetic amplifier method and the magnetic multi-vibrator method, the detecting circuit and the lead wire being detected can be insulated electrically. However, as previously described, the core made of the soft magnetic material must be magnetically deflected by the DC current flowing through the lead wire, so as to be saturated substantially near to the saturated magnetic flux density (Bs). When using a known soft magnetic material such as a permalloy as the core, and for example, the electric current flowing through the lead wire being detected is about several tens of mA, the lead wire must be wound around the soft magnetic material core with several tens to several hundreds of turns or more. Thus, originally, it is difficult to use such a method as the DC current sensor for the leakage breaker and the like requiring one through turn of the lead wire. Also in the hall device method, the detecting capability is inevitably decided by characteristics of the hall device. When previously known hall devices are used, and for example, the electric current flowing through the lead wire is about several tens of mA, the lead wire must be wound around the soft magnetic material core above several hundreds to several thousands of turns. Thus, similar to the magnetic amplifier method and the magnetic multi-vibrator method, it was difficult for such a device to be used as the DC current sensor for the leakage breaker and the like requiring one through turn of the lead wire. We are also aware of US 4 280 162 A which is the basis of the respective preamble of claims 1 and 6. SUMMARY OF THE INVENTION It is object of the present invention to provide a sensitive DC current sensor, which solves the above-mentioned problems, having a relatively simple construction and a good detecting capability for a DC leakage breaker and the like, particularly, against a microscopic current variation. It is, particularly, another object of the present invention to provide a versatile DC current sensor, which is very simple to mount to a lead wire. The inventors have noticed that, when a lead wire is extended through a detecting core consisting of an annular soft magnetic material, around which a detecting core is wound in a toroidal shape, and a DC current is applied therethrough, a clockwise magnetic field is generated against the direction of the DC current and a magnetic flux Φ 0 is produced in the detecting core. Since the electric current flowing through the lead wire is a DC current, the magnetic flux Φ 0 is constant and an electromotive force is not produced in the detecting coil. The inventors also found that an electromotive force can be produced in the detecting coil by forming a magnetic gap partly in the detecting core, which is open and closed by a magnetic body to constitute a magnetic switch, and changing (ON-OFF) the magnetic flux Φ 0 with time by the magnetic switch. Furthermore, as the result of various studies to make the above-mentioned configuration more realistic, the inventors have confirmed that this object can be achieved by disposing means for partly forming the magnetic gap periodically in the detecting core which provides a magnetic flux generated in a substantially perpendicular direction, against the magnetic flux generated in a circumferential direction in the detecting core by the DC current flowing through the lead wire. This can be used in place of a mechanical magnetic switch, yet realising the operation substantially similar to the aforesaid magnetic switch. Specifically, it can be constituted by disposing an exciting core consisting of a soft magnetic material connected perpendicularly to the circumferential direction of a detecting core to form an annular shape. An exciting coil is wound on a portion of the exciting core in a toroidal shape. Further, the exciting core is excited in a perpendicular direction against the circumferential direction of the detecting core by periodically applying a DC current to the exciting coil to magnetically saturate the intersection of the detecting core and the exciting core the magnetically saturated intersection is formed substantially into the magnetic gap. That is, because the fact that a relative permeability µ of the magnetically saturated intersection of the detecting core approaches to 1 endlessly, the magnetically saturated portion functions the same as the magnetic gap. The magnetic flux Φ 0 in the detecting core reduces at a constant period and the electromotive force can be produced in the detecting coil with changes in the magnetic flux. The inventors have made various improvements on the DC current sensor consisting of the aforementioned basic configuration, and in order to mount the DC current sensor simply to the lead wire particularly, in such a way that the detecting core can be divided, at least, at one portion in the circumferential direction at the time of disposing the lead wire therethrough, a plurality of detecting core members are used and the exciting core, exciting coil and detecting coil are disposed effectively on one of the detecting core members to achieve the object. That is, the DC current sensor of the present invention comprises a DC current sensor as recited in claim 1 or claim 6. By integrating the exciting core, exciting coil and detecting coil effectively with one of the detecting core members, interconnection between the other detecting core members is not necessary, thus it is easy to mount to the lead wire. The possibility of breaking the wire at mounting can be largely reduced, thus a reliable mounting of the DC current sensor can be realised. Since the DC current sensor of the present invention has a good detecting capability against a microscopic current variation as shown in the embodiment, when used in the DC leakage breaker and the like, a required sensitive detection can be achieved by extending only one lead wire through the detecting core. Thus the construction is relatively simple and the DC current sensor can be made smaller. Besides, not only an absolute value of the DC current flowing through the lead wire can be measured, but also its direction so that it can be used more effectively, for example in the control of an actuator using a DC motor. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective explanatory view showing an outline of one embodiment of a DC current sensor leading to the present invention. Fig. 2 is a longitudinal section explanatory view taken along a line a-a of the DC current sensor shown in Fig. 1. Fig. 3 is a longitudinal sectional explanatory view taken along a line b-b of the DC current sensor shown in Fig. 1. Fig. 4 is a perspective explanatory view showing an outline of another embodiment of a DC current sensor leading to the present invention. Fig. 5 is a longitudinal sectional explanatory view taken along a line a-a of the DC current sensor shown in Fig. 4. Fig. 6 is a longitudinal sectional explanatory view taken along a line b-b of the DC current sensor shown in Fig. 4. Fig. 7 is a graph showing the relationship between an exciting current, a magnetic flux passing a detecting core and an electromotive force of a detecting coil in a DC current sensor configuration shown in Fig. 4. Fig. 8 is a schematic explanatory view showing a mode at the time of using the DC current sensors shown in Fig. 1 and Fig. 4. Fig. 9 is a linear graph showing the relationship between a DC current flowing through a lead wire and an output of the DC current sensor shown in Fig. 1. Fig. 10 is a perspective explanatory view using modification of the DC current sensor shown in Fig. 1. Fig. 11 is a perspective explanatory view showing the principle of a DC current sensor of the present invention. Fig. 12 is a graph showing the relationship between frequencies applied in a DC current sensor configuration shown in Fig. 10, wherein Fig. 12(A) shows the relationship between the frequency and an exciting current; Fig. 12(B) shows the relationship between the frequency and a magnetic flux passing a detecting core, and Fig. 12(C) shows the relationship between the frequency and an electromotive force of a detecting coil. Fig. 13 is a graph showing the relationship between frequencies applied in the DC current sensor configuration shown in Fig. 10, wherein Fig. 13(A) shows the relationship between the frequency and an exciting current, Fig. 13(B) shows the relationship between the frequency and a magnetic flux passing a detecting core, and Fig. 13(C) shows the relationship between the frequency and an electromotive force of a detecting coil. Fig. 14 is a perspective explanatory view of an embodiment of the principle of a DC current sensor of the present invention. Fig. 15 is a perspective explanatory view of an embodiment of the principle of a DC current sensor of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, the operation of a DC current sensor of the present invention is particularly described based on the drawings. As previously described, the DC current sensor of the present invention comprises a detecting core which consists of a plurality of detecting core members so as to be divided, at least at one portion, in a circumferential direction of the detecting core at the time of disposing a lead wire therethrough. For the purpose of describing the details on completion of the present invention more clearly, in the following description, the basic operational principle is, first, described based on the configuration in which the detecting core is not divided. Fig. 11 is a perspective explanatory view for explaining the basic principle of operation of a DC current sensor of the present invention. Fig. 12 and Fig. 13 show the relationship between an exciting current and a magnetic flux passing a detecting core and an electromotive force produced in a detecting coil in this configuration. In Fig. 11, numeral 1 designates a lead wire disposed through the detecting core 2 consisting of an annular soft magnetic material. Numeral 3 designates a detecting coil wound around a predetermined portion of the detecting core 2 in a toroidal shape, and connected to a predetermined detecting circuit (not shown) as securing an electrical insulation with the lead wire 1. Numeral 4 designates an exciting core consisting of the annular soft magnetic material, around which an exciting coil 5 is wound in a toroidal shape at a predetermined position. The exciting core 4 is connected perpendicularly to the detecting core 2 at a portion in a circumferential direction thereof, and by the operation to be described later, forms an magnetically saturated portion at a shaded portion in the figure, corresponding to a core intersection 6 of the detecting core 2 and the exciting core 4. In the figure, numeral 43 designates a modulation coil which is disposed in the same direction as the lead wire 1, for the purpose of reducing an output characteristic hysteresis which is assumed to be caused by the effect of magnetic characteristic (coercive force) of the soft magnetic material constituting the detecting core 2. In the configuration shown in Fig. 11, when a DC current I flows through the lead wire 1, a clockwise magnetic field is generated in the detecting core 2 against the direction of the DC current I. Thereby, a magnetic flux Φ 0 is produced in the detecting core 2. At this time, when a predetermined AC current is applied to the exciting coil 5 it produces a magnetic flux, which changes in a direction α periodically, in the exciting core 4. This magnetically saturates the exciting core 4 periodically, a relative permeability µ drops nearly to 1, and the core intersection 6 (a shaded portion) which is a circumferential portion of the detecting core 2 substantially forms a so-called magnetic gap, and the magnetic flux Φ 0 in the detecting core 2 is reduced to Φ 1 . When the exciting core 4 is saturated near a peak value of the AC current of frequency f 0 applied to the exciting coil 5, both in the case of DC current I flowing through the lead wire 1 in a positive (+) direction (upward in the figure) as shown in Fig. 12, and in the case of the DC current I flowing through the lead wire 1 in a negative (-) direction (downward in the figure) as shown in Fig. 13, the exciting core 4 is saturated twice in one exciting current period. In the case of positive (+) direction (upward in the figure) of the DC current I flowing through the lead wire 1 as shown in Fig. 12, by this saturation, the magnetic flux Φ 0 produced in the detecting core 2 by the DC current I flowing through the lead wire is reduced to Φ 1 at the frequency of 2f 0 as shown in Fig. 12(B). That is, modulation takes place at 2f 0 . Thus, a voltage V DET of frequency 2f 0 is produced in the detecting coil 3, as shown in Fig. 12 (C), as the magnetic flux changes. Also, in the case that DC current I flowing through the lead wire 1 is in the negative (-) direction (downward in the figure) as shown in Fig. 13, though the operation is substantially same as the case of positive (+) direction (upward in the figure) of the DC current I, due to the fact that the direction of the DC current I is in reverse, the direction of the magnetic flux produced in the detecting core 2 is also in reverse and a phase difference of the voltage V DET of frequency 2f 0 produced in the detecting coil 3 becomes 180°. However, regardless of the direction of the DC current I flowing through the lead wire 1, in either case, voltage V DET α DC current I from the relationship of magnetic flux Φ 0 α DC current I and voltage V DET α magnetic flux Φ 0 , thus the electromotive force proportional to the DC current I flowing through the lead wire 1 can be detected by the detecting coil 3, and an absolute value of the DC current I flowing through the lead wire 1 can be determined. Furthermore, since the detecting core 2 and the exciting core 4 are connected perpendicularly, the exciting magnetic flux in the exciting core 4 does not leak to the detecting core 2 and pass through the detecting coil 3. The electromotive force generated by the exciting current applied to the exciting coil 5 is not produced in the detecting coil 3, so that when the DC current I flowing through the lead wire 1 is 0, V DET = 0. Since the frequency of the electromotive force V DET produced in the detecting coil 3 is 2f 0 and different from the frequency f 0 of the exciting current applied to the exciting coil 5, even when the exciting magnetic flux in the exciting core 4 leaks due to the accuracy of shapes, sizes and so on of the detecting coil 2 and the exciting coil 4, and is detected by the detecting coil 3, the frequency of a leakage component can be easily separated by a frequency discriminating filter and the like. Thus, it can be used as a sensitive DC current sensor. As a result of study made on the DC current sensor having the above-mentioned configuration to enable detection of the microscopic currents, it has been confirmed that, a so-called hysteresis phenomenon, where an output voltage (electromotive force) in the detecting coil is different, is produced, even at the same current value when the DC current increased or reduced, in a microscopic current region, where the DC current flowing through the lead wire 1 is, for example, about ± 50 mA or less. This occurs even when using permalloy C (78% Ni-5Mo-4Cu-balFe) known as a material of a high relative permeability as the detecting core 2. In particular, when the DC current is around zero (± 20 mA), an "inversion region", where the output voltage is reduced as the DC current is increased, is generated. That is, when the DC current (current being detected) flowing through the lead wire 1 is at zero, even when the above-mentioned material is used as the detecting core 2, since the coercive force of the material is finite, the magnetic flux remains in the core, and until the residual magnetic flux is eliminated by the current being detected, the "inverse region" according to the hysteresis phenomenon of the output voltage in the detecting coil in the microscopic current region is produced, and a reference level at the time of measurement fluctuates. Thus at the measurement in the microscopic current region, measurement values differ at each time and an accurate value can not be obtained. In order to solve such problems, as shown in Fig. 11, the modulation coil 43 wound in the same direction as the lead wire 1 is disposed on the detecting core 2, and an alternating magnetic field produced by the modulation coil 43 is superposed on the detecting core 2, thereby the hysteresis of the output characteristic by residual magnetic flux of the core can be reduced. Accordingly, when an AC current necessary to produce the magnetic field higher than a coercive force of the detecting core 2 is applied to the modulation coil 43, the hysteresis characteristic produced by the residual magnetic flux caused by the coercive force of the core material is eliminated, and a detecting sensitivity at a microscopic current can be enhanced by removing the superposed alternating component by the detecting circuit. The modulation coil 43 is wound around the detecting core 2 in the same direction as the lead wire 1 such that, besides winding by one turn around the detecting core 2 so as to extend in the same direction as the lead wire 1 as the embodiment shown in Fig. 11, it is wound by several turns in the same direction as aforementioned responsive to a necessary intensity of the alternating magnetic field. Particularly, in the case of winding by several turns, it is wound around the detecting core 2 in a toroidal shape as substantially same as the detecting coil 3. As it is apparent from Fig. 11, since the modulating coil 43 is disposed substantially at the same location in the same direction as the detecting coil 3, which is wound around the detecting core 2 in a toroidal shape, the detecting coil 3 may be commonly used. That is, since the frequency of the electric current flowing through the detecting coil 3 and that of the electric current flowing through the modulation coil 43 are, originally, different from one another largely, even when they are used commonly, an electric signal can be separated easily by arranging a filter, which passes the electric current composed of the frequency realizing respective functions. Thus even when the modulation coil 43 and the detecting coil 3 are integrated, the output characteristic hysteresis can be reduced. Meanwhile, viewing the fact that the phase difference of the voltage V DET of frequency 2f 0 produced in the detecting coil 3 depending on the direction of the DC current I flowing through the lead wire 1 is 180° as described by Fig. 12 and Fig. 13, it has been confirmed that, an absolute value as well as the direction of the DC current flowing through the lead wire 1 can be detected easily, by applying an exciting current in a state, where the frequency of the exciting current generated from an oscillator at the frequency of two times the exciting current is divided into half, and detecting the phase difference of the oscillator output and the detecting coil 3 output by a phase comparing circuit. That is, since both the frequency of the exciting current generated from the oscillator connected to the exciting coil 5 and that of the output V DET from the detecting coil 3, have a frequency 2f 0 which is two times the exciting current applied to the exciting coil 5, the phase difference can be easily compared. Thus the absolute value as well as the direction of the DC current flowing through the lead wire 1 can be detected. Likewise, by applying an electric current, whose direction and intensity change linearly against time periodically, for example an electric current which changes in a triangular waveform, to the detecting coil 3 to give a deflection magnetic field in the detecting core 2, the absolute value as well as the direction of the DC current flowing through the lead wire 1 can be easily detected. That is, when the electric current which changes, for example, in a triangular waveform, is applied to the detecting coil 3 in the state where the DC current flows through the lead wire 1, a magnetic flux produced by the triangular waveform electric current and a magnetic flux produced by the DC current flowing through the lead wire 1 are superposed in the detecting coil 3. Thus the absolute value as well as the direction of the DC current flowing through the lead wire 1 can be detected by, after applying a crest restriction to the electromotive force produced in the detecting coil 3, detecting the output phase and detecting a time ratio (duty ratio) between the positive (+) side output time and the negative (-) side output time. Particularly, when a maximum value of the aforesaid electric current applied to the detecting coil 3 is sufficiently set to produce the magnetic field above a coercive force (αHc) of the material of the detecting core 2, the output characteristic hysteresis substantially caused by a hysteresis of the material of the detecting coil 2 can be reduced. Furthermore, by combining various known electric circuits effectively, essential advantages of the DC current Sensor constituted as shown in Fig. 11 can be practically used more effectively. Meanwhile, based on the above-mentioned configuration, a pair of cylinders constituting an exciting core are disposed in parallel against the axis line, and adjoining sides of opening ends of the cylinders are connected in a body with connecting plates consisting of a soft magnetic material, to constitute the detecting core by the connecting plates and cylinder side faces connected thereto, thereby an electromagnetic imbalance of the DC current sensor and also noise is reduced. The S/N ratio can therefore be improved. Also, by adopting the configuration of the exciting core constituted by a pair of cylinders disposed in parallel against the axis line, as compared with the configuration shown in Fig. 11, a width d (refer to Fig. 11) of a connection of the exciting core connected to the detecting core can be substantially lengthened. As a result, a ratio of width d of the connection of the exciting core against a magnetic path length of the detecting core (a magnetic gap ratio) is increased, a residual magnetic flux density in the detecting core is reduced by the effect of a demagnetizing field, and the effect of coercive force of the core material is reduced further by geometrical effects with the arrangement effect of the modulation coil. For example, a DC current sensor constituted as shown in Fig. 14 includes the aforesaid effects, whereby a stable measurement can be realized. That is, in the configuration shown in Fig. 11, though the electromagnetic balance of the DC current sensor is difficult to accomplish, because the exciting core 4 is connected to the detecting core 2, and also the detecting coil 3, exciting coil 5 and modulation coil 43 are respectively disposed at one location, the DC current sensor shown in Fig. 14 takes into consideration the electromagnetic balance of the exciting core 4, detecting coil 3, exciting coil 5 and modulation coil 43. - In Fig. 14, numeral 1 designates a lead wire, which extends through the inner centre portion of a detecting core 2 having a rectangular frame shape. On long side portions at opposite positions of the rectangular frame-shaped detecting core 2, a pair of exciting cores 4a, 4b are disposed in a body so as to form a quadrangular cylinder. Meanwhile, at side faces of the outermost peripheries of the pair of exciting cores 4a, 4b, exciting coils 5a, 5b are wound in a toroidal shape. A pair of detecting coils 3a, 3b are wound around the pair of exciting cores 4a, 4b in a toroidal shape for electrical connection. Furthermore, on short side portions at opposite positions of the rectangular frame-shaped detecting core 2, a pair of modulation coils 43a, 43b are disposed to extend through in the same direction as the lead wire being detected 1, and are connected in series electrically by predetermined means. When a DC current I flows through the lead wire 1 in such a configuration, a magnetic field, which is clockwise to the direction of the DC current I, is produced in the detecting core 2, and a magnetic flux Φ 0 is generated in the detecting core 2. At this time, when a predetermined AC current is applied to the exciting coils 5a, 5b to produce a magnetic flux, which changes in a direction a periodically in the pair of exciting cores 4a, 4b, which are saturated magnetically, periodically, the core intersection 6 at the long side portion, which is a part of circumferential direction of the rectangular frame-shaped detecting core 2, substantially forms a so-called magnetic gap where a relative permeability µ is extremely close to 1, thereby the magnetic flux Φ in the detecting core is reduced to Φ 1 . Thus, the above-mentioned DC current sensor also has a same mechanism of producing the electromotive force to the pair of detecting coils 3a, 3b as the configuration shown in Fig. 11, and the effect of the mechanism is similar. Furthermore, in this configuration, the effect of disposing the pair of modulation coils 43a, 43b, the effect of reducing the residual magnetic flux density in the detecting core 2 resulting from the demagnetizing effect by increasing the ratio of width d of the connection of the exciting core 4, against the magnetic path length of the detecting core 2, and the effect of electromagnetic balance resulting from the overall symmetrical configuration against the lead wire 1 are obtained. Meanwhile, the inventors have studied a DC current sensor in which the above-mentioned configuration was further improved. Though precise detection is possible in any of the configurations of the DC current sensor described above, since the detecting core 2, through which the lead wire 1 is disposed, is formed into a body, it is very complicated to mount to the lead wire. That is, in the case of mounting to the lead wire, it is necessary to cut off a power source and disconnect the wiring to dispose the lead wire 1 through a through hole of the detecting the core 2. A DC current sensor shown in Fig. 15 is constituted to solve such problems whereby it can be mounted to the active lead wire and to the wiring where the lead wires are difficult to cut and connect. A basic configuration of the DC current sensor shown in a perspective explanatory view of Fig. 15 is similar to the DC current sensor having the configuration shown in Fig. 14 previously described. That is, numeral 1 designates a lead wire which is disposed through an inner centre portion of a detecting core 2. In this configuration, however, the detecting core 2 is divided into a pair, and at end portions of detecting core members 2a, 2b, L-shaped mounting members 60a, 60b consisting of the same material as the detecting core members 2a, 2b are secured, and integrated by means of a screw 61, after disposing the lead wire 1 at a predetermined position. A pair of exciting cores 4a, 4b are disposed in a body on respective opposite long side portions of the detecting core members 2a, 2b as to form a quadrangular cylinder. Meanwhile, around side faces of the utmost periphery of the pair of exciting cores 4a, 4b, exciting coils 5a, 5b are wound in a toroidal shape. Furthermore, detecting coils 3a, 3b are also wound around the exciting cores 4a, 4b in a toroidal shape. Modulation coils 43a, 43b are wound at the same locations as the detecting coils 3a, 3b. When a DC current I is applied to the lead wire 1 in such a configuration, by the same mechanism as the configuration shown in Fig. 14, the DC current flowing through the lead wire 1 can be detected by the detecting coils 3a, 3b. Also, substantially the same effect as the DC current sensor constituted as shown in Fig. 14 can be realized. In the configuration shown in Fig. 15, taking basically the electromagnetic balance into consideration, from the fact that the detecting coils 3a, 3b, exciting coils 5a, 5b and modulation coils 43a, 43b are wound around the divided detecting core members 2a, 2b independently, when the DC current sensor is mounted to the lead wire 1 in the exciting equipments, the lead wire 1 is surrounded by the detecting core members 2a, 2b so as to dispose the former substantially through the latter without being cut. Thereafter, the detecting core members 2a, 2b are integrated, and the detecting coils 3a, 3b, exciting coils 5a, 5b and modulation coils 43a, 43b are connected. Such connections are, in fact, very complicated, and in the configuration where the connections are completed, there is the possibility of cutting the wires when disposing the lead wire 1, requiring careful handling more than necessary. In order to solve such problems, for example, in the DC current sensor constituted as shown in Fig. 15, only one detecting coil 3a, 3b, exciting coils 5a, 5b and modulation coils 43a, 43b wound around the divided detecting core members 2a, 2b and exciting cores 4a, 4b may be used. That is, it may be constituted such that, the exciting core 4b, detecting coil 3b, exciting coil 5b and modulation coil 43b are not disposed on one detecting core member 2b, but the exciting core 4a, detecting coil 3b, exciting coil 5a and modulation coil 43 are disposed only on another detecting core member 2a, thereby a predetermined electromotive force can be outputted to the detecting coil 3a, by the same mechanism as described before, by exciting the exciting coil 5a. However, in the DC current sensor shown in Fig. 15, the respective cores and coils are disposed by considering the electromagnetic balance. Thus the above-mentioned coil arrangement may break the electromagnetic balance, besides, an exciting signal by the exciting coil 5a is mixed largely into the detecting coil 3a to make the precise detection impossible. Particularly, a second high frequency of the exciting signal caused by non-linearity of the magnetic characteristic originally included in the soft magnetic material constituting the detecting core 2a and the exciting core 4a is mixed into the detecting coil 3a, besides, since the second high frequency and a detecting signal (an electromotive force of the detecting coil 3a) have the same frequency, it is impossible to separate them electrically. Thus, in the above-mentioned configuration, it was difficult to provide the DC current sensor having a high S/N ratio, which is easy to handle the versatile. The DC current sensor of the present invention has been developed by, as it is apparent from the details of various improvements described heretofore, adopting the configuration shown in Fig. 11 as the basic configuration, and developing the configuration considering the electromagnetic balance as shown in Fig. 14, and developing the configuration which can be mounted to the existing lead wire without cutting the same as shown in Fig. 15 to widen the versatility of the DC current sensor constituted as shown in Fig. 14, and further, improving disadvantages of the configuration shown in Fig. 15 for solving the aforementioned problems, whereby the detecting core is constituted by a plurality of detecting core members so as to be divided, at least, at one portion when disposing the lead wire 1 therethrough, and the exciting core, exciting coil and detecting coil are disposed effectively on one of the detecting core members. The specific configuration is particularly described by the embodiments shown hereinbelow. Fig. 1 is a perspective explanatory view showing one embodiment of a DC current sensor leading to the sensor of the present invention, Fig. 2 is a longitudinal sectional view taken along a line a-a of Fig. 1, and Fig 3 is a longitudinal sectional view taken along a line b-b of Fig. 1. In the figure, numeral 1 designates a lead wire carrying a current to be detected, which is disposed inside a detecting core 2 constituting a rectangular frame. The detecting core 2 is constituted by a detecting core member 2a, which is formed by connecting I-shaped detecting core members 21a, 21b by a portion of an exciting core 4 to define a body, and a U-shaped detecting core member 2b which is formed into a body beforehand. The exciting core 4 is constituted by 4 substantially rectangular cylindrical exciting core members 4a, 4b, 4c, 4d, which are oppositely disposed via the I-shaped detecting core members 21a, 21b. Opposite portions 52a, 52b, 52c, 52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b are formed in a body by connecting the detecting core members 21a, 21b, and constitute a portion of the detecting core member 2a. That is, the opposite portions 52a, 52b, 52c, 52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b form a core intersection of the exciting core, which is connected substantially orthogonally to the circumferential direction of the detecting core member. In the figure, numerals 5a, 5b designate a pair of exciting coils, which are wound around adjoining portions 51a, 51b and 51c, 51d of the exciting core members 4a, 4b, 4c, 4d. Meanwhile, the pair of exciting coils 5a, 5b are connected for negative-phase exciting of the respective exciting coils 5a, 5b. Numeral 3 designates a detecting coil, which is wound in a body around the exciting core members 4a, 4b, 4c, 4d in a toroidal shape. When the pair of exciting coils 5a, 5b are subjected to negative-phase excitation in such a configuration, magnetic fluxes Φ 11 , Φ 12 , Φ 13 , Φ 14 produced in the exciting core members 4a, 4b, 4c, 4d by the exciting coils 5a, 5b occur as shown in Fig. 2. Hereupon, when a DC current I is applied to the lead wire 1, a magnetic flux Φ 0 is produced in a circumferential direction of the detecting core 2 constituted by the pair of detecting core members 2a, 2b, and also in the opposite portions (core intersections of the exciting cores) 52a, 52b, 52c, 52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b described previously, the magnetic flux Φ is produced in the same direction (circumferential direction). The magnetic fluxes Φ 11 , Φ 12 , Φ 13 , Φ 14 produced by the negative-phase excitation of the pair of exciting coils 5a, 5b are, substantially, in the perpendicular direction against the magnetic flux Φ 0 in the circumferential direction produced by the DC current flowing through the lead wire 1, at the opposite portions 52a, 52b, 52c, 52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b, thereby a magnetic path by the magnetic flux in the circumferential direction is interrupted by the portions periodically, and a required output can be obtained in the detecting coil 3 by the same electromagnetic force producing mechanism as the DC current sensor constituted as shown in Fig. 11, Fig. 14 and Fig. 15 previously described. However, since the pair of exciting coils 5a, 5b are subjected to negative-phase excitation in this configuration, the magnetic fluxes Φ 11 and Φ 13 and Φ 12 Φ 14 produced in the exciting core members 4a and 4c, 4b and 4d which are disposed oppositely via the detecting core members 21a, 21b, are in the opposite direction with each other against the winding direction of the detecting coil 3, and as same as a leakage magnetic flux produced by excitation of the exciting coils 51, 5b, the second high frequency of an exciting signal is also substantially rejected. As a result, an exciting signal can be prevented from mixing into the detecting coil 3. Particularly, by adopting the configuration, in which a balance volume 70 (refer to Fig. 1) is disposed between the pair of exciting coils 5a, 5b to balance the exciting current applied to the exciting coils 5a, 5b in such a manner that, the maximum effect of the magnetic fluxes Φ 11 and Φ 13 , Φ 12 and Φ 14 (refer to Fig. 2) can be obtained, or a residual output from the detecting coil 3 becomes minimum, the S/N ratio can be improved further. Fig 4 is a perspective explanatory view showing another embodiment of a DC current sensor leading to a current sensor of the present invention. Fig. 5 is a longitudinal sectional view taken along a line a-a of Fig. 4, and Fig. 6 is a longitudinal sectional view taken along a line b-b of Fig. 4. Also in this DC current sensor, the configuration of the detecting core 2 and the exciting core 4 is substantially similar to that of the DC current sensor shown in Fig. 1. That is, in the figure, numeral 1 designates a lead wire which is disposed inside the detecting core 2 constituted substantially in a rectangular frame. The detecting core 2 is constituted by a detecting core member 2a, which is formed by connecting I-shaped detecting core members 21a, 21b by a portion of the detecting core 4 to be described later in a body, and a U-shaped detecting core member 2b which is a preformed body. The exciting core 4 is constituted by 4 substantially rectangular cylindrical exciting core members 4a, 4b, 4c, 4d, which are oppositely disposed via the I-shaped detecting core members 21a, 21b, and opposite portions 52a, 52b, 52c, 52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b are formed in a body by connecting the detecting core members 21a, 21b to constitute a portion of the detecting core member 2a. That is, the opposite portions 52a, 52b, 52c, 52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b form a core intersection of the exiting core, which is connected substantially orthogonal to the circumferential direction of the detecting core member. In Figures 4 and 5, numerals 5a, 5b designate a pair of exciting coils which are wound around adjoining portions 51a, 51b and 51c, 51d of the exciting core members 4a, 4b, 4c, 4d. The pair of exciting coils 5a, 5b are so connected that, exciting currents whose relative phase difference is 90° (phase difference of 90° or 270°) can be applied. For example, they are connected to a power source via a 90° phase circuit as shown at numeral 71 (refer to Fig. 4). In Figures 4 and 5, numerals 3a, 3b designate detecting coils, which are wound in a toroidal shape around the exciting core members 4a, 4b, around which the exciting coil 5a is wound, and the exciting core members 4c, 4d, around which the exciting coil 5b is wound. The pair of detecting coils 3a, 3b are connected in series in a negative phase. When exciting currents having a relative phase difference of 90° are applied to the paid of exciting coils 5a, 5b in such a configuration, as shown in Fig. 5, magnetic fluxes Φ 21 , Φ 22 and Φ 23 , Φ 24 are produced in the exciting core members 4a, 4b and 4c, 4d based on the phase difference of the exiting currents. However, even though the magnetic flux is produced in the same direction in the exciting core members 4a and 4c, 4b and 4d, due to the phase difference of the exciting current in the coils, when the exciting core members 4a, 4b are excited, the exciting core members 4c, 4d are not excited. Hereupon, when the DC current I is applied to the lead wire 1, a magnetic flux Φ 0 is produced in a circumferential direction of the detecting core 2 constituted by the pair of detecting core members 2a, 2b, and also at opposite portions (core intersection of the exciting core) 52a, 52b, 52c, 52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b previously described, the magnetic flux Φ is produced in the same direction (circumferential direction). Also, the magnetic fluxes Φ 21, Φ 22, Φ 23, Φ 24 produced by applying the exciting current, whose relative phase difference is 90°, to the pair of exciting coils 5a, 5b are substantially in the perpendicular direction against the magnetic flux Φ 0 produced in the circumferential direction by the DC current flowing through the lead wire 1, in the opposite portions 52a,52b, 52c,52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b. Thus a magnetic path by the magnetic flux in the circumferential direction is interrupted periodically by the portions. However, as described above, due to the phase difference of the exciting currents, at the opposite portions 52a, 52b, 52c, 52d of the exciting core members 4a, 4b, 4c, 4d facing the detecting core members 21a, 21b, while the magnetic path by the magnetic flux in the circumferential direction is interrupted by the opposite portions 52a, 52b, the opposite portions 52c, 5d are connected without interrupting the magnetic path. Describing further in detail, for example, when the exciting current shown by a solid line in Fig 7(A) is applied to the exciting coil 5a, in a state where the magnetic flux Φ 0 is produced in the circumferential direction in the detecting core 2, the magnetic flux Φ 0 reduces periodically to Φ 1 as shown by a solid line in Fig. 7(B), by the magnetic fluxes Φ 21 , Φ 22 produced at the opposite portions 52a, 52b of the exciting core members 4a, 4b facing the detecting core members 21a, 21b. Likewise, as shown by broken lines in Fig. 7(A), when the exciting current, whose phase difference is 90° relative to the exciting current applied to the exciting coil 5a, is applied to the exciting coil 5b, the magnetic flux Φ 0 is periodically reduced to Φ 1 as shown by broken lines in Fig. 7(B), by the magnetic fluxes Φ 23, Φ 24 produced at the opposite portions 52c, 52d of the exciting core members 4c, 4d facing the detecting core members 21a, 21b. That is, in the detecting coils 3a and 3b, responsive to the magnetic flux variations shown by the solid and broken lines in Fig. 7(B), by the same electromotive force producing mechanism as the DC current sensor constituted as shown in Fig. 11, Fig. 14 and Fig. 15 described previously, predetermined outputs as shown by the solid and broken lines in Fig. 7(C) are obtained. However, when the pair of detecting coils 3a, 3b are connected in a positive phase, as it is apparent from Fig. 7(C), the electromotive forces of the detecting coils 3a, 3b cancel one another, and the required output characteristic can not, substantially, be obtained. However, when the detecting coils 3a and 3b are connected in series in a negative phase, an electromotive force as shown by a two-dot chain line in Fig. 7(C) responsive to the magnetic flux variation as shown by a two-dot chain line in Fig. 7(B) is substantially produced in the detecting coil 3b, as a result, a sum of the electromotive forces of the detecting coils 3a, 3b is outputted. Besides, since the detecting coils 3a, 3b are connected in a negative phase, even when foreign noises are mixed thereinto, the noises cancel one another and only the electromotive force produced by the DC current flowing through the lead wire 1 is output accurately. Furthermore, in the DC current sensor constituted as such, since the relative phase difference of the exciting current applied to the exciting coils 5a, 5b is 90°, and the magnetic flux Φ 0 in the circumferential direction produced in the detecting core 2 by the DC current flowing through the lead wire 1 substantially maintains, as previously described, a constant value without being interrupted as viewing from the detecting core 2 as a whole (even when interrupted partly, it is connected through the other portion), a fall of output due to deterioration of the alternating magnetic characteristic of the detecting core 2 material can be prevented. That is, when a magnetic flux produced in a longitudinal direction of the opposite portions 52a, 52b is Φ ab , and a magnetic flux produced in the longitudinal direction of the opposite portions 52c, 52d is Φ cd , Φ 0 = Φ ab + Φ cd , and Φ ab + Φ cd = Φ 0 from Fig 7(B), which is constant, thus the magnetic flux produced the pair of detecting core materials 2a, 2b does not change and substantially maintains a constant value proportional to I. Thus, the detecting core members 2a, 2b are just required to be made of a material having a good DC magnetic characteristic, and the alternating magnetic characteristic may be neglected. Hence, for example, a permalloy block material which is advantageous in strength can be used. The DC current sensor constituted as shown in Fig. 1 and Fig. 14 is arranged so that the detecting coil 3, exciting core 4 and exciting coil 5 are disposed in a body only on one of the pair of detecting core members 2a, 2b which can be divided. Thus, it is not necessary to connect to the other detecting core members. Thus it is easy to mount to the existing lead wire, thereby the possibility of breaking wires at installation is considerably reduced and the DC current sensor can be mounted reliably. Fig. 8 is a schematic explanatory view showing a configuration of detecting core members 2a, 2b used in such a manner that, the DC current sensor constituted as shown in Fig. 1 and Fig. 4 can be handled easier. In the figure, numeral 81a designates a support member for supporting and securing the detecting core member 2a, and 81b designates: a support member for supporting and securing the detecting core member 2b, which are both constituted by a bakelite plate or a synthetic resin plate having an electrical insulation and consisting of a non-magnetic material. Meanwhile, glass epoxy plates 82a, 82b are bonded to the outer faces of the support members 81a, 81b to improve the mechanical strength thereof, and to secure the electrical insulation between the detecting core 2, detecting coil 3, exciting core 4, exciting coil 5 and the outside. Furthermore, these components are entirely covered with a shield case 83 consisting of an anisotropic silicon steel plate or the like to prevent induced noises from mixing in. In practical use, in order to avoid improper electrical connection at butt faces of the detecting core members 2a, 2b, for example the butt faces are polished to secure a high flatness, and in order to maintain a good connection, it is desirable to dispose various known fixing means on the support members 81a, 81a. The DC current sensor of the present invention is such that various configurations can be adopted. For example, the modulation coil 43 which is disposed in the configurations in Fig 11, Fig. 14 and Fig 15 is not shown in the configurations shown in Fig. 1 and Fig. 4 but is in fact an essential feature of the present invention. Also, by adopting the configuration in which the modulation coil 43 and the detecting coil 3 are used commonly in a body, the output characteristic hysteresis can be reduced. Since the absolute value as well as a direction of the DC current flowing through the lead wire can be detected easily, means for applying the exciting current in a state, where the frequency of the exciting current generated from an oscillator, beforehand, at the frequency of two times the exciting current is divided into half, to the exciting coil 5 to detect the phase difference of the oscillator output and the detecting coil output by a phase comparing circuit, or means for applying the electric current whose direction and intensity change linearly periodically against the time, for example, change in a triangular waveform to the detecting coil 3, to give a modulation magnetic field in the detecting core may be adopted. Furthermore, by combining various known electric circuits effectively, essential advantages of the DC current sensor of the present invention can be used more effectively. In the DC current sensor of the present invention, the soft magnetic material constituting the detecting core and the exciting core are desirably selected responsive to the intensity of electric current flowing through the lead wire, or the detecting sensitivity required for the sensor. Soft magnetic materials such as a silicon steel plate, amorphous, electromagnetic soft iron and soft ferrite may be used independently or in combination. The magnetic material may just be constituted to form an electromagnetically closed circuit, and besides the rectangular frame-shaped detecting core and the rectangular cylindrical exciting core as shown in the figure, various configurations such as a circular or oval ring-shaped detecting core may also be adopted. Particularly, as to the detecting core, since it is constituted by a plurality of detecting core members so as to be divided, at least, at one portion in the circumferential direction when disposing the lead wire therethrough, it is desirable to devise the configuration of the detecting core member, which is, finally, easy to assemble. As with the aforesaid embodiment, it is also possible to use a portion of the exciting core as a member constituting the detecting core member. When considering the final assembling for integration, it is desirable that the detecting core is divided as little as possible and the detecting core members are reduced as much as possible. It is also possible to use the invention as a clamp meter by adopting the configuration as shown in Fig. 10. Fig. 10 shows the configuration in which the detecting core materials 2a, 2b of Fig. 1 are improved. That is, depressions and protrusions are respectively formed at opposite portion of end portions of the I-shaped detecting core members 21a, 21b constituting the detecting core member 2a, which is connected with the exciting core 4, and end portions of the U-shaped detecting core member 2b which is preformed for engagement. Furthermore, one of the engaged portions (the side of detecting core member 21b) is held by a screw 90 so that the other engaged portion (the side of detecting core member 21a) is opened and closed about the screw 90. Though the above-mentioned depressions and protrusions may be formed by machining the opposite portions of the detecting core members 21a, 21b and 2b made of a thick plate, they may be formed more easily by constituting the detecting core members 21a, 21b and 2b proper by laminating thin plates having different lengths. The opposite portions of the detecting core members 21a, 21b, 2b are not necessarily limited to the configuration of depressions and protrusions, the configuration, in which the respective faces are finished precisely to butt the flat faces one another, may be selected responsive to the required measurement accuracy. The configuration described above can be similarly adopted in the DC current sensor constituted as shown in Fig. 4. In the DC current sensor of the present invention, with respect to the magnetic saturation at core intersections between the detecting core and the exciting core, for example, even when the core intersections are not completely saturated, as far as the saturated state can be substantially obtained, the required detection can be accomplished. Thus, by selecting optimum conditions including, the aforesaid soft magnetic material, its shape and size and turns of the detecting coil and exciting coil, a sensor which can be used more practically can be provided. Meanwhile, in either of the aforementioned configurations, a single lead wire is disposed through the detecting core. A plurality of lead wires may also be disposed responsive to the size of the required sensor. For example, when two lead wires are disposed and the direction of the electric current flowing through the two lead wires is different, it is possible to output only a difference of electric current value flowing through the two conductors. Thus the effect of the DC current sensor of the present invention can be realized most effectively when used in leakage detection of the DC equipments. Example Example 1 Four pieces of rectangular cylindrical exciting core members 4a, 4b, 4c, 4d were prepared, by punching a thin plate of 0.35 mm thick composed of permalloy C (78% Ni - 5% Mo - 4% Cu - balFe) into predetermined shapes, which were bent at predetermined locations and assembled for spot welding. A pair of I-shaped detecting core members 21a and a U-shaped detecting core member 2b were also punched from a thin plate of 1.5 mm thick having the same composition as the exciting core member. Then, the four exciting core members 4a, 4b, 4c, 4d were spot welded together with the pair of I-shaped detecting core members in a body, and further, these component members and the U-shaped detecting core member 2b were heated in a hydrogen gas atmosphere at 1100°C for 3 hours, thereafter the heat treatment was performed through multi-stage cold treatment between 400°C and 600°C at 100°C/hour. A core assembly shown in Fig. 1 was obtained in the above-mentioned manner. Where, appropriate length L = 26 mm, height H = 7.5 mm and width W = 27 mm. After winding an insulation protective vinyl tape around adjoining portions of the rectangular cylindrical exciting core members, a wire of 0.3 mm outside diameter was wound by 1 turn to obtain exciting coils 5a, 5b, which were connected for negative-phase excitation. Also, after winding the insulation protective vinyl tape around the exciting core members, the wire of 0.1 mm outside diameter was wound by 300 turns to obtain a detecting coil 3. The detecting coil 3 was combined with a predetermined filter circuit to use a modulation coil commonly. In such a manner, the DC current sensor shown in Fig. 1 was completed. Meanwhile, the detecting core members 2a, 2b of the DC current sensor were disposed and connected in a body around a lead wire 1 of 8 mm outside diameter consisting of a vinyl coating, and finally, the lead wire 1 was disposed through the detecting core 2. Fig.9 shows the output variations after removing an AC component of 11 Hz, by a low-pass filter, in an electromotive force (output) V out of the detecting coil 3 outputted via a phase comparing circuit, when disposing AC current applying means, in which an oscillator producing an exciting current having a frequency two times the exciting current applied finally to an exciting coil 5 is disposed, the phase comparing circuit and the like on the exciting coils 5a, 5b and detecting coil 3 of the present DC current sensor, applying the AC current of f = 3.5 kHz and 0.1 Amps as the exciting current to the exciting coils 5a, 5b for negative-phase excitation, applying a sine wave AC current of 11 Hz and 1mA (at peak) to allow the detecting coil to function as the modulation coil of the oscillator, and further applying a DC current I to the lead wire 1 in a range of ± 100 mA. The output voltage Vout is a value outputted via an amplifying circuit having an amplifying effect, and for the purpose of understanding the effect of the present invention clearly, the output is enlarged in the vicinity where the DC current I of the lead wire 1 is ± 10 mA. As it is apparent from the above measurement result, according to the present DC current sensor, even when the detecting core can be divided, an error output due to a reciprocating current is very small, and even by a microscopic current of 10 mA, the measurement can be effected at a S/N ratio of 10 or more and the sensitive measurement is possible. Example 2 The core assembly of the embodiment of Fig. 4 was used. After winding an insulation protective vinyl tape around the adjoining portions of the rectangular cylindrical exciting core members, a wire of 0.3 mm outside diameter was wound by 15 turns to obtain the exciting cores 5a, 5b, which were connected to a power source via a predetermined phase circuit so as an exciting current having the phase difference of 90° is applied to the exciting coils 5a, 5b. After winding the insulation protective vinyl tape around the exciting core members disposed oppositely via the detecting core members 21a, 21b, the wire of 0.1 mm outside diameter was wound by 150 turns to obtain the detecting coils 3a, 3b, which were connected in series in a negative phase. The detecting coils 3a, 3b were combined with a predetermined filter circuit to use a modulation coil commonly. In such a manner, the DC current sensor shown in Fig.4 was completed. Meanwhile, the detecting core members 2a, 2b constituting the DC current sensor were disposed and connected in a body around the lead wire 1 of 8 mm outside diameter consisting of a vinyl coating, and finally, the lead wire 1 was disposed through the detecting core 2. As the result of measurement on output variations after removing an AC component of 11 Hz, by a low-pass filter, in an electromotive force (output) Vout of the detecting coil 3 outputted via a phase comparing circuit, when disposing current applying means, in which an oscillator producing an exciting current having the frequency two times the exciting current applied finally to the exciting coils 5a, 5b is disposed, the phase comparing circuit and the like on the exciting coils 5a, 5b and detecting coils 3a, 3b as the present DC current sensor, applying the exciting current (AC current) of f = 3.5 kHz and 0.1 Amps to the exciting coils 5a, 5b for the phase difference of 90°, applying a sine wave Ac current of 11 Hz and 1 mA (at peak) to allow the detecting coils 3a, 3b to function as the modulation coil of the oscillator, and further applying a DC current I to the lead wire in a range of 100 mA, it has been confirmed that the approximately same measurement result as that shown in Fig. 9 can be obtained. That is, similar to the DC current sensor shown in Fig. 1, also in the DC current sensor constituted as shown in Fig. 4, even though the detecting core can be divided, the error output due to the reciprocating current is very small and the sensitive measurement is possible.

Claims: A DC current sensor comprising: a detecting core (2) comprising a plurality of core members of soft magnetic material formed in such manner as to provide an electromagnetically closed circuit adapted to encircle a lead wire (1) for carrying a DC current flow, the sensor being adapted to detect said flow without contacting the wire (1); an exciting core (4) defining a closed magnetic circuit connected with one of the members of the detecting core (2) to define a detecting core intersection perpendicular to the circumferential direction of the detecting core members; and a detecting coil (3); characterised by : the exciting core comprising two pairs of hollow rectangular tubular exciting core members (4a, 4b, 4c, 4d) of soft magnetic material, the pairs being arranged on opposite sides of the one detecting core member; two exciting coils (5a, 5b) which are connected in opposite phases and are respectively wound around adjoining portions (51a, 51b, 51c, 51d) of a respective one of the pairs of the exciting core members (4a, 4b, 4c, 4d), the coils carrying exciting currents of opposite phase thereby to magnetically saturate a portion of the one of the detecting core members (2a, 2b) by a magnetic flux perpendicular to the magnetic flux (Φ 0 ) which is in the circumferential direction and produced by the current flowing through the lead wire (1) and to interrupt periodically the magnetic path of the magnetic flux in the circumferential direction, and the detecting coil (3) being wound around the one member of the detecting core (2) and also wound around the exciting core members (4a, 4b, 4c, 4d) perpendicular to the exciting coils in a toroidal manner; and a modulation coil (43) wound around the one detecting core member in the same direction as the detecting coil; the detecting core (2) being: rectangular frame-shaped and constituted by a plurality of core members (2a, 2b) interconnected so that at at least one location the members (2a, 2b) are releasable circumferentially to allow insertion of the lead wire (1) which is thereafter encircled by the core (2). A DC current sensor according to claim 1 characterised in that the detecting core (2) comprises a detecting core member (2a) formed by connecting a pair of I-shaped detecting core members (21a, 21b) by a portion (52a, 52b, 52c, 52d) of the exciting core (4), and the core (2) further comprises a detecting core member (2b) preformed in a U-shape. A DC current sensor according to Claim 2 characterised in that the detecting core members (2a, 2b) are hinged at one end (90) and at the other end the members releasably interengage. A DC current sensor according to Claim 1 characterised in that balance circuitry (70) is provided for balancing the exciting current applied to the exciting coils (5a, 5b). A DC current sensor according to any one of the preceding claims 1 to 4 characterised in that the detecting core (2) and the exciting core (4) are formed of permalloy. A DC current sensor comprising: a detecting core (2) comprising a plurality of core members of soft magnetic material formed in such manner as to provide an electro-magnetically closed circuit adapted to encircle a lead wire (1) for carrying a DC current flow, the sensor being adapted to detect said flow without contacting the wire (1); an exciting core (4) defining a closed magnetic circuit connected with one of the members of the detecting core (2) to define a detecting core intersection perpendicular to the circumferential direction of the detecting core members; characterised by : the exciting core comprising two pairs of hollow rectangular tubular exciting core members (4a, 4b, 4c, 4d) of soft magnetic material, the pairs being arranged on opposite sides of the one detecting core member; two exciting coils (5a, 5b) which are energised with exciting currents having a relative phase difference of 90 degrees and are respectively wound around adjoining portions (51a, 51b, 51c, 51d) of associated core members of a respective pair of exciting core members (4a, 4b, 4c, 4d) thereby to magnetically saturate a portion of the one of the detecting core members (2a, 2b) by a magnetic flux perpendicular to the magnetic flux (Φ 0 ) which is in the circumferential direction and produced by the current flowing through the lead wire (1) and to interrupt periodically the magnetic path of the magnetic flux in the circumferential direction, and two detecting coils (3a, 3b), each wound around a respective one of the pairs of exciting core members (4a, 4b, 4c, 4d) perpendicular to the exciting coils in toroidal manner, the detecting coils being connected in opposite phases; the detecting core being rectangular frame-shaped and constituted by a plurality of core members (2a, 2b) interconnected so that at at least one location the members (2a, 2b) are releasable in a circumferential direction to allow insertion of the lead wire (1) which is thereafter encircled by the core (2); and a modulation coil (43) wound around the one detecting core member in the same direction as the detecting coils. A DC current sensor according to Claim 6 characterised in that the detecting core (2) comprises a detecting core member (2a) formed by connecting a pair of I-shaped detecting core members (21a, 21b) by a portion (52a, 52b, 52c, 52d) of the exciting core (4), and the core (2) further comprises a detecting core member (2b) preformed in a U-shape. A DC current sensor according to Claim 7 characterised in that the detecting core members (2a, 2b) are hinged at one end (90) and at the other end the members releasably interengage. A DC current sensor according to Claim 6 characterised in that a 90° phase circuit (71) is disposed between the exciting coils (5a, 5b). A DC current sensor according to Claims 6 to 9 characterised in that the detecting core (2) and the exciting core (4) are formed of permalloy.

Inventors: KAWAKAMI MAKOTO, YAMAGUCHI SHIGERU, KAWAKAMI, MAKOTO, YAMAGUCHI, SHIGERU
Applicants: NEOMAX CO LTD, NEOMAX CO., LTD.
Application year: 1994
Main classification: G01R15
IPC codes: G01R15, G01R19